1. Field of the Invention
This invention relates to heating vessels and, in particular, to an apparatus for and method of adjusting the effective length of a rotary kiln as it is used to preheat batch material in a glass melting operation, but is applicable to other processes that require the heating of generally flowable materials.
2a. Technical Considerations
It has long been recognized that exhaust gas from combustion processes contains thermal energy that can be recovered to improve the overall efficiency of the process. In particular, glass melting furnace or other melting process furnaces contain large amounts of recoverable thermal energy. Conventionally, regenerators and recuperators have been employed to recover heat from melting process furnaces by preheating combustion air to be used during the melting process, but their efficiency is less than desired. Instead of preheating combustion air by way of regenerators or recuperators, it has been proposed to recover waste heat by means of preheating the feed material.
A rotary kiln can be used as a first stage preheating vessel in a multi-stage melting process. Exhaust gas from a second stage melting furnace is directed through the kiln to heat the material. The length of the kiln is chosen in accordance with the amount of heat to be transferred from the hot exhaust gases to the feed material. With all other factors remaining constant, the longer the kiln, the more time the feed material is exposed to the hot exhaust gases. When feed material must be exposed to the exhaust gas for a longer time than possible with a given kiln configuration due to the nature of the batch components or variations in the melting process in the second stage furnace, the kiln must be modified to add additional sections to increase its length in order to increase the overall residence time of the feed material in the kiln.
Dusting of dry pulverulent batch material is a problem when feeding the batch materials to a conventional melting furnace. A common solution to this problem is to wet the batch (e.g. with water), but preheating the batch to any significant extent precludes maintaining the batch in a wetted condition. Furthermore, air turbulence created by preheating gases exiting the rotary kiln at high velocities tend to contribute to the entrainment of the dry batch materials in the gases as it is loaded into the kiln. The dusted exhaust gas must then be filtered to remove the air particulates for pollution purposes.
In U.S. Pat. No. 4,381,934 to Kunkle and Matesa, there is disclosed an intensified batch liquefying process in which large volumes of batch are efficiently liquefied in a relatively small space. This type of process, particularly when using intensified heat sources, produces relatively small volumes of high temperature exhaust gas that can be used to preheat batch material in a rotary kiln and thus further improve the overall efficiency of the process. In a process such as disclosed in U.S. Pat. No. 4,381,934, the firing rate, i.e. the amount of heat used in the second stage varies, depending on the specific operating conditions and parameters. The varying firing rate inherently varies the exhaust gas temperature and flow which in turn affects the preheating conditions within the rotary kiln. It would be desirable to develop a way to modify the effective length of a heating vessel, so as to provide the proper heating length that corresponds to the varying heat content of exhaust from a 2nd stage heating process. In addition, it is desirable to use the hot exhaust gas while limiting the amount of dust entrainment within the vessel.
2b. Patents of Interest
U.S. Pat. No. 2,176,267 to Meiselman et al. teaches a sweating type furnace with two coaxial drums. Material is charged into the inner drum and as the furnace rotates, the charge progresses towards the lower end. A burner at the discharge end melts specific metals in the charge which, in turn, drop through slots in the inner drum to the outer drum for subsequent collection.
U.S. Pat. No. 2,857,684 to Haldorsson teaches a rotary cooler and dryer with a slidable exhaust duct. The bell-shaped mouth of the sliding duct is positioned within the rotary dryer drum at the area of deepest suction within the dryer so that dust particles of a certain size can be separated from the rest of the material and be discharged from the cyclone.
U.S. Pat. No. 4,427,376 to Etnyre et al. teaches a rotary dryer for aggregate material wherein a burner at the discharge end directs a flame through an elongated firing tube toward the dryer loading end. The hot gas strikes a baffle plate and is deflected forward through a chamber between the firing tube and an outer drum towards the discharge end. As a result, the material within the drum is heated indirectly by the gas passing through the firing tube and directly by the reflected gas flowing back to the discharge end.
U.S. Pat. No. 4,519,814 to Demarest, Jr. teaches as a two stage liquefaction process wherein batch material in the first stage is heated to an intermediate temperature by exhaust gas from the second stage heater. In order to reduce dust entrainment caused by the exhaust gas blowing over and through the batch material, the temperature of the exhaust gas is reduced below the dew point of water by sufficient countercurrent contact between the gas and the material. This wetted condition reduces particulates in the exhaust stream.